Letters

Sunday, May 01, 2005

A safe exposure to mercury?“Our Preferred Poison” [March] is a comprehensive and well-researched exposition of mercury. The American Dental Association speaks out of both sides of its mouth, claiming that mercury in dental fillings is inert when combined with the alloy powder while admitting that mercury vapor is emitted from amalgam fillings—but in such small quantities that it causes no harm. The general population has been left with the impression that fish and coal-burning plants are the primary sources of human exposure. However, in 1990 the World Health Organization concluded that mercury in dental fillings was the primary source of mercury exposure. A later document also concluded that it was not possible to set a “safe” level of exposure, due to the difficulty of obtaining data. A number of peer-reviewed studies have affirmed this conclusion. As a lawyer who has tried over 100 mercury poisoning cases since 1978, I can tell you that there is no substantial evidence to support a conclusion that any level of mercury exposure is safe. The concern about the toxicity of mercury is reflected by the steep decline in the use of mercury in industry over the past two decades. The real story here is why an exception has been made for the dental industry.Sandra DuffyLake Oswego, Oregon

Karen Wright’s article stated that “A single drop [of mercury] on a human hand can be irreversibly fatal.” While that may be true of some of the element’s compounds, when you say “mercury,” you are speaking of free-state mercury, and it simply isn’t true. I started teaching over 40 years ago, when less was known of mercury’s effects. I heated mercuric oxide to show the silvery lining of the test tube; I must have made barometers and manometers about 150 times, touching the element each time. I have also had a dental amalgam filling since I was about 10 years old. I’m 63 years old now and fairly athletic. If the dangerous amalgam idea is valid, why wouldn’t a person with so many other contamination experiences be more affected?Laymond H. NorthHampton, Virginia

The toxicity of mercury is subtle and varies widely depending on the circumstances of exposure. For skin contact, a single drop of dimethyl mercury can indeed be fatal, and the poison is the mercury, not the material added to mercury to create dimethyl mercury. Metallic or free-state mercury gives off vapors so toxic that the Environmental Protection Agency regards the breaking of a thermometer worthy of a nine-step cleanup effort. Spills of even two tablespoons must be handled by a hazardous-waste response team. If it seems that many of us have escaped unharmed from such exposure, remember that the symptoms of vapor poisoning can be as subtle as muscle weakness, headache, grumpiness, and difficulty sleeping. Neurological deficits induced by low-level exposure are not overtly pathological. Your reflexes may be a bit slower, or it may take longer to recall a name. Bear in mind, too, that exposure that poses little risk for an adult can threaten the health of a young or unborn child. Individuals may also differ in their ability to excrete mercury. So even though you may not be suffering any consequences from repeated mercury exposure, someone else could be. —Karen Wright

A human paradoxRegarding March’s “Human, Study Thyself,” Part I of Discover’s Learning Series on Genes, Race, and Medicine: Geneticist Georgia Dunston claims that the physical characteristics of races are superficial but advocates more medical research on black subjects. The two positions are inconsistent, as is her statement “We are all the same, and we are all unique.” Her claim that “there’s no direct correlation between a racial group and anything in the human genome” is false. Everything about race correlates directly to the human genome. The major races are different, on the average, in almost every anatomical, physiological, and biochemical characteristic. Her statements, positions, and motivation appear to be political, not scientific.George L. ClarkManhattan Beach, California

These are not simply my claims: The preponderance of scientifically verified biological data do not support the substructure of human populations into any discrete, internally consistent racial subgroups. When sampled adequately, the genetic differences in populations dissolve into a continuum of variation. The partitioning of humans into biological races was permissible when the knowledge of our genetic inheritance was based on less than 0.1 percent of the human genome. However, based on data now available, we see that the sequence of the 3 billion nucleotides in any individual genome is unique in comparison with the sequence of another individual’s genome, while the degree of sequence similarity between the 3 billion nucleotides in any two genomes is remarkably high. The uniqueness of the individual human genome in the presence of extreme similarity between any two genomes challenges the concept of human races. The reader implies that a scientist cannot be both passionate and objective about the same subject. Would he make a similar claim about the many distinguished scientists who have been advocates for political and social change, such as Albert Einstein? Since policy issues are influenced by scientists, I am indeed passionate about the need for the biomedical research community to be vigilant in its responsibility to correctly interpret and communicate its findings. If this is what the reader is referring to when he says that my “statements, positions, and motivation are political, not scientific,” then so be it. —Georgia Dunston

The good plaqueAlthough I enjoyed your brief etymological history of the word plaque in the March issue [Delineations, R&D], you forgot one important current and historical use. Since medieval times, players of double-reed instruments in the oboe, bassoon, shawm, dolcian, and chanter family have used a small, thin, usually arrow-shaped or semicircular piece of metal, now usually plastic or wood, to help scrape and shape the cane used to construct the reed. A plaque, slipped between the two thin pieces of cane, helps support the tip of the reed while a musician makes his or her daily adjustment to it. The usage fits right in alongside the Flemish idiom for a small coin and the French word for a thin, flat patch.Peter BrowerSaint Louis Park, Minnesota

ErrataIn “The Grandest Rocket Ever” [February], we state that “at its peak” the Orion project had “a budget of more than $2 million (the equivalent of $1.28 billion today).” Two million dollars in the peak year of 1962 would be about $12.8 million today. In March’s “Drilling San Andreas,” we erred in stating that “the San Andreas Fault curves up 800 miles from the Gulf of Mexico.” The fault curves northward from the Gulf of California.